It is well known that thin metallic and ceramic layers may be deposited upon a substrate by a technique known as "sputtering". By such methods, a metal layer may be sputtered in an argon atmosphere from a target of the material to be deposited, generally as a cathode in a standard RF and/or DC sputtering apparatus.
More recently, sputtering techniques have been used in the production of integrated circuits which require rapid and economical metal deposition with metal line widths and spaces less than 1 micron. Sputtering is an especially useful tool for the application of films and coatings where uniformity and chemical purity are important. Manufacturing costs may be lowered by improving film uniformity and deposition rate in high speed production processes typically used in integrated circuit manufacturing. Materials of particular importance in the manufacture of integrated circuits are aluminum, copper, titanium, tungsten, and their alloys. Targets of these materials are used to produce a metal or intermetallic film or coating on a substrate by sputtering.
Illustrative sputtering processes and apparatus with which the invention may be usable are disclosed in Bergmann, et al., U.S. Pat. Nos. 4,889,772 and 4,961,831; Shagun, et al., U.S. Pat. No. 4,961,832; Shimamura, et al., U.S. Pat. No. 4,963,239; Nobutani, et al., U.S. Pat. No. 4,964,962; Arita, U.S. Pat. No. 4,964,968; Kusakabe, et al., U.S. Pat. No. 4,964,969 and Hata, U.S. Pat. No. 4,971,674; and the references referred to therein; sputtering targets are discussed also in Fukaswawa, et al. U.S. Pat. Nos. 4,963,240 and 4,966,676; and Archut, et al., U.S. Pat. No. 4,966,676. These disclosures of sputtering processes and apparatus as well as sputtering targets are expressly incorporated herein by reference.
Given the importance of high deposition rates and film uniformity to economical production of high quality integrated circuits, investigations have been undertaken to consider the relationship between the nature of the sputtering target and the nature of the resulting deposited layer.
It is believed there are three parameters of target structure which are related to sputtering effectiveness. One factor is grain size since, rather than being one continuous crystal structure, solid metals are typically composed of separate and discrete grains of continuous crystal lattice. Depending on the composition and forming method of the metal, these grains can vary in size from the millimeter range to the micron range. Target grain size is also important to achieve high deposition rates and uniform deposited layers. Targets with fine grain sizes also enable higher deposition rates than targets with larger grain sizes because discontinuities at grain boundaries are more readily attacked during sputtering. Correlations have been found between grain size of targets and uniformity of deposited layers as shown in FIG. 1.
A second, more important, factor is the crystallographic orientation of the grains. Each grain is a continuous crystal, with its crystal lattice oriented in some particular way relative to a reference plane such as the sputtering surface of the target. Since each grain is independent of the others, each grain lattice has its own orientation relative to this plane. When grain orientation is not random, but when crystal planes tend to be aligned in some way relative to a reference plane, the material is said to have "texture". These textures are denoted using standard indices which define directions relative to crystallographic planes. For instance, a target made from a metal with cubic crystal structure, such as aluminum or copper, may have a &lt;100&gt;, a &lt;110&gt; or other textures. Similarly, a target made from a metal with hexagonal crystal structure, such as titanium, may have a &lt;0002&gt; texture. The exact texture developed will depend on the metal type and the work and heat treatment history of the target. The effect of crystallographic orientation of a sputtering target on sputtering deposition rate and film uniformity has been described in an article by C. E. Wickersham, Jr., entitled Crystallographic Target Effects in Magnetron Sputtering in the J. Vac. Sci. Technol. A5(4), Jul./Aug. 1987 publication of the American Vacuum Society. In this article, the author indicates that improvements in film uniformity may be achieved on a silicon wafer by controlling the working process for making a target.
A third parameter, applicable to alloy targets, is the size of those regions in the target which comprise a second phase constituent rather than the matrix metal. Although a portion of the alloying element is dissolved into the matrix material, some "precipitate" may be distributed throughout the matrix. Certain precipitates are associated with particulates during sputtering, which may cause yield losses during the manufacture of integrated circuits. Minimizing precipitate size can also affect sputtering performance of alloy targets.
However, there is a limit to how fine a grain size, how strong a texture, and how small a precipitate size can be achieved with conventional metal processing techniques for each metal system and alloy. For example, in the case of aluminum, it is common for a target to have a grain size of 100 microns to 1 millimeter with a less-thanoptimum crystallographic orientation. Grain sizes may be reduced by using grain refiners such as titanium diboride, but these materials should not be present in sputtering targets because they contaminate the sputtered deposit. Those alloy elements which may be desirable components of a sputtered deposit do not have a sufficient grain refining effect to produce optimum target grain size.
To improve the performance of sputtering targets, manufacturers have used special casting techniques to reduce the resulting as-cast grain size. Also, deformation followed by recrystallization has been used to reduce the grain size of the metal to be formed into a sputtering target.
Grain orientation control has also been suggested. A slow hot forging technique which produces a predominately &lt;110&gt; texture is described in U.S. Pat. No. 5,087,297 to Pouliquen.
Conventional casting, forming, annealing, and forging techniques have produced sputtering targets with limited minimum grain sizes such as is set forth in Table I below:
TABLE I ______________________________________ Typical Minimum Grain Sizes from Conventional Metal Working Techniques Minimum Minimum Conventional Conventional Target Metal Grain Size Precipitate Size ______________________________________ Aluminum, .about.100 microns .about.10 microns Aluminum Alloys Titanium .about.10 microns N/A Copper .about.30 microns N/A ______________________________________
Metals with relatively small grains have been produced by a technique known as liquid dynamic compaction (LDC), but not in the production of sputtering targets. Porosity levels traditionally associated with spray forming methods would suggest that such methods are not suitable for sputtering target manufacture since porosity is completely undesirable. The LDC method can be adapted to produce fine grain size and very low porosity levels but prior to this invention there had been no use of the LDC process for making targets even though, as now discovered, the single step LDC process is lower cost than processes used heretofore for target manufacture. LDC involves gas atomizing molten metal to produce a fast moving spray of liquid metal droplets. These droplets splat quench upon impact with a substrate. Fragmentation and fast cooling nucleate small crystals to yield fine grains with ultra fine dendritic structure. As the atomized metal is sprayed onto a substrate, a highly dense (greater than 99% dense) metal product can be formed with grain size approximately one to two orders of magnitude smaller than produced by other conventional powder production methods. It is also possible to keep precipitate sizes below 1 micron. For example, in aluminum alloys, grain sizes less than 10 microns and precipitate sizes of less than 1 micron may be obtained. LDC is also the only known method capable of producing sputtering targets (high density targets with low oxygen content) with random texture, which may be valuable for step coverage and via fill.
Ultra-fine grains have also been achieved with a technique known as equal channel angular extrusion (ECAE), but not in production of sputtering targets. Prior to the present invention, the ECAE process has been a technical curiosity but has not been used for any known commercial purpose. Even experienced aluminum extruders considered such a process to be non-commercial and beyond their realm of experience. ECAE is a method which uses an extrusion die containing two transversely extending channels of substantially identical cross section. It is common, but not necessary, to use channels which are perpendicular to each other, such that a cross section of the transverse channels forms an "L" shape.
In this technique, a well lubricated workpiece of metal, usually in the form of a plate, is placed into one of the channels. The workpiece cross section is substantially identical to the channel cross section, so that the workpiece fits tightly into the channel. A punch then forces the workpiece to exit the die through a second contiguous transverse channel. As the workpiece is forced through the corner formed by the contiguous channels, it moves through as a rigid material, and deformation is achieved by simple shear in a thin layer at the crossing plane of the channels. This shearing and subsequent heat treatment is effective in reducing grain size in the workpiece metal to approximately 2 to 3 orders of magnitude smaller than the other currently available methods for achieving small grain size.
Although structural applications have been proposed for metals formed by these two techniques, neither LDC nor ECAE have been used in the formation of sputtering targets. In accordance with the invention these techniques are applied with appropriate materials in a manner which creates sputtering targets with improved grain sizes, textures, and precipitate sizes. Use of such improved targets results in improved sputtering deposition rates and sputtered film quality.